Method of and means for television transmission



M y 1940- HANNS-HEINZ WOLFF METHOD OF AND MEANS FOR TELEVISION TRANSMISSION Ffiled Dec. 4, 1936 Patented May 14, 1940 UNITED" STATES METHOD OF AND MEANS FOR TELEVISION TRANSMISSION Harms-Heinz Wolff, Berlin, Germany, assignor, by

mesne assignments, to Loewe Radio, Inc., a corporation of New York Application December 4, 1936, Serial No. 114,161

. In Germany December 10, 1935 9 Claims.

The invention relates to a method of and means for television transmission, making use of cathode ray tubes, in which a storage of light-electric excitations and the initiation of current impulses corresponding to the stored excitations by cathode ray scanning or the like take place. Arrangements of this nature have been developed according to the point of view of raising, by way of storage, the image-content impulses employed for transmission purposes to sufliciently The invention is based on the idea that if too great a length of time elapses between illuminating and scanning, the electric image on the light-electric plate varies owing to mutual influence of the different charged image elements, especially in the sense of softening of contrast. This will occur even in those cases in which by very careful insulation a compensation of the charge by surface leakage currents is to alarge extent suppressed. A compensation of the charge is caused in part by capacitative effect of the different image elements one upon another, and in part by the reactionary effects of space charges, which form in front of the image elements, are

determined, as regards their strength and spacial extent in the first place by the individual charges, and which have the tendency to compensate each other and then by way of a reactionary effect alter the electrical image.

According to the invention, therefore, care is taken that the interval of time between completion of the illumination of an individual image element and its scanning by the cathode ray has practically thesame value in respectof all image elements. Care is preferably also taken that this interval of time is as short as possible. Additional features of the invention are set forth in the following description and explanation in conjunction with the accompanying drawing. Of

w the drawing Fig. 1 shows diagrammatically the parts of the total arrangement essential for a form of embodiment of the invention, whilst in v 4 Figs. 2 and 50-0 is shown the image areaor image field for illustrating certain considerations important for the invention, in

Fig. 3 a diaphragm arrangement not in accordance with the invention. and in each of Figs. 4 and 6 a diaphragm arrangement accord- 5 ing to the invention.

The intermittent uncovering and interruption of the passage of the ray from the object to be transmitted, or a part thereof, to the light-electrically sensitive plate, or a corresponding part thereof, may be performed by, means ofa rotating diaphragm. The opening or openings in this diaphragm which uncover or partially uncover the passage of the ray must appear sharply on the light-electrically sensitive plate simul- 15 taniously with the object of reproduction. The diaphragm should accordingly be arranged if possible either immediately in front of the lightelectrically sensitive plate or immediately in front of the object of reproduction. The former ar- 20 rangement is only. possible if the light-sensitive plate is situated sufficiently near the wall of the cathode ray tube. The latter arrangement can be performed only in the case of image or film transmissions, and then only with difiiculty. In 25 very many cases it will not be possible to fulfill either of these two requirements. According, therefore, to an additional feature of the in vention the procedure will be such that the object of reproduction is first reproduced sharply on the diaphragm, and then the diaphragm together with the real image of the object of reproduction reproduced sharply on the light-sensitive plate. The diagram of this arrangement is shown in Fig. 1. In the same, I is the object 5 to be reproduced, 2 the cathode ray tube with the light-sensitive plate 3, 4 the diaphragm having the axis of rotation 5, B a reproducing lens, which projects an image l of the object I on to the diaphragm 4, and 8 a second reproducing lens which reproduces the image I, so far as it is uncovered by the diaphragm, on the plate 3 at l.

In Fig. 2 the image I (l) is shown as being divided into m lines each having n image elements, i. e., altogether into mm image elements the cathode first column are again shaded after being illuminated, and be t2=t1+At the time at which the cathode ray starts scanning the image area i at the point an. The difference in time between shading and scanning accordingly amounts, for the image point an, to At. The point am is shaded at the same moment as the point an, but is scanned by the cathode ray by the amount later than the point am, assuming that is represents the image-scanning period in seconds.

The arrangement in Fig. 3 accordingly shows disadvantages which are to be obviated by the method according to the invention, as in this arrangement there is no usein-providingAt with a value which is small in relation to the image scanning period tb. An arrangement according to the invention is shown in Fig. 4. In this figure the arrangement is such that a shading edge ill of a diaphragmaperture moves over the image Tin such fashion that it runs parallel to the lines along which the cathode ray scanning takes place thatthe movement of said edge occurs perpendicularly to the direction of the lines in the sense ofthe sequence in which the lines are scanned by ray. In this arrangement the greatest difference between the time intervals between the shading and the cathode ray scanning of the individual image elements is evidently approximately equal to one line period, which inmany cases may already represent a sufficient degree of conformity.

In this connection the arrangement should be such that the edge it! of the diaphragm aperture passes over the image I within the same period which is required by the cathode ray for scanning the image T. If the cathode ray has scanned the entire image, the line can am must in the meantime have been illuminated anew and already again shaded before the oathode ray starts the next scan. It follows that'the diaphragm t'requires to be furnished with a rim of-evenly. distributed apertures H. The dimensions and the distribution of these apertures are determined by the following points of view:

In the case of film transmission with intermittent advance of the film there must be pro- .vided between two apertures an intermediate space of such nature that the entire image field is shaded during a certain fraction, necessary for the advance of the film, of the complete operating period tg. Be this fraction assumed to amount, for example, to flltg. It would be de sirable in itself for the cathode ray to start the next scan (at the point an) practically immediately after scanning the element amn- This, however, is not feasible. To enable the ray to start the new'scan upon its arrival at thelower edge of the image it would be necessary, when it reaches the lower edge of the image, for a shading of the entire image field during the time interval I and an illumination and renewed shading of the first line to have already taken place. The complete shading would accordingly require to commence at a time when the cathode ray is situated ata distance from the lower edge-of the image which corresponds with the period of time if ii is assumed to be the duration of illiunination as determined by the width of the diaphragm aperture. The ray scanning, however, would then require to lag behind the darkening to the extent of t -i+At+n i. e., there would result the contradiction At=%+t;i-At

The ray scanning accordingly requires to be interrupted for the duration i. e., in the assumed case that the time is required for. the advance of the film, of the timeavailable for. one scan the time interval i i-Hz so that a loss of time available for the cathode ray scanning is occasioned practically by the advance of the film alone.

An additional reason for making the width of the slot as small as possible consists in the fact that if the slot is too wide, the charges of an image element undergo already during the illumination detrimental variations such as the invention seeks to avoid. It is reasonable to select t1 and At to be of the same order.

The last aspect referred to is also decisive in the direct transmission of moving occurrences, in which the loss of 'timeby advance of a film is not involved. The ratio between the mutual distance of two neighbouring slots and the width of a slot amounts to hi ii If the'disc 4 possesses 5 slots ll distributed about its'periphery, the same, according'to-a' further feature-of the invention, must-have a circumferential velocity which is equal to times-the number of image changes per second, i. e., for example in the case of 25 image changes per second and 25 slots revolution I second In the case of rapidly moving occurrences which are directly transmittedthe arrangement according to the invention has a further important advantage, which will be explained with reference to Figs. 5111-0. Be a bright red assumed to move over the image field I ('l) on a dark ground, e. g.,

during-a complete image-scanning period from the position [2 into the position [3 according to Fig. 50.. Upon omission of the above described diaphragm or analogous means this results in an image of the form of the triangle l4 (see Fig. 5b), since until the scanning cathode ray has reached, for example, the line of the image indicated by the broken line, all image elements of this line situated within the area id have received light excitations corresponding to the light intensity of the rod. If on the other hand only one or a few lines are illuminated at a time, and that shortly before they are scanned by the cathode ray, there results as image substantially the confinement l5 of the triangle [4. A moved image of this nature, therefore, however appearing distorted according to Fig. 50, does not appear in blurred form as shown in Fig. 519. It is evident that the distortion represents a considerably less serious drawback than the blur.

The advantages to be obtained by the invention are achieved to a still greater extent than in the arrangement describedabove by the following arrangement.

The diaphragm 4 is constructed as a spiral aperture disc on the lines of the Nipkow disc (see Fig. 6). The apertures are made in the form of longer or shorter slots or in the extreme case as openings each uncovering only one image element at a time, dependent on the extent to which it is desired to make use of the storage efiect or on whether it is desired to dispense with the same for the sake of a more complete elimination of the disturbances which the invention is intended to reduce. The width of the slots is adapted to the width of the scanning lines and their number is selected to be equal to the number of lines. The circumferential velocity of the disc per second is in this case equal to the complete number of image scannings per second.

In order still further to eliminate even the distortion described in conjunction with Figs. 541-0 there may be employed in accordance with the invention a simple or preferably multiple interlaced line method, the number of image scannings per second being doubled or preferably tripled, generally multiplied with corresponding reduction of the number of lines per image scanning operation. According to a subsidiary feature of the invention the diaphragm 4 is then given a form which corresponds to that of the scanning discs of television transmitters operating according to the interlaced scanning method (compare for example the applications Ser. No. 36008/35 filed Aug. 13, 1935), attention being merely called to the fact that the openings may again have the form of slots extended in the direction of the lines for the purpose of obtaining a certain storage effect. In this form, therefore, the arrangement according to the invention combines the following important advantages:

(1) Variations in charge are avoided by a particular selection of the duration of illumination and the chronological order of illumination and scanning.

(2) Blurring effects in the transmission of rapidly moving occurrences are avoided by brevity of the illumination of the individual image element, and

(3) Distortions in the transmission of rapidly moving occurrences are avoided by the use of a, preferably multiple, interlaced line method.

I claim:

1. In a television transmitter comprising a cathode ray tube having a photo-electric mosaic upon which the image to be transmitted is projected and which is scanned by a cathode ray, means to illuminate each element of said mosaic only for a small part of an image change period and to scan it immediately thereafter, said means consisting of an obturating diaphragm rotating synchronously with the scanning, said diaphragm having a plurality of elongated apertures being so arranged as to uncover at once at least one of the image lines.

2. In a television transmitter comprising a cathode ray tube having a photo-electric mosaic upon which the image to be transmitted is projected and which is scanned by a cathode ray, means to illuminate each element of said mosaic only for a small part of an image change period and to scan it immediately thereafter, said means consisting of an obturating diaphragm rotating synchronously with the scanning, said diaphragm having a spiral of apertures similar to a Nipkow disc.

3. In a television transmitter comprising a cathode ray tube having a photo-electric mosaic upon which the image to be transmitted is projected and which is scanned by a cathode ray, means to illuminate each element of said mosaic only for a small part of an image change period and to scan it immediately thereafter, said means consisting of an obturatlng diaphragm rotating synchronously with the scanning, saiddiaphragm having a spiral of apertures similar to a Nipkow disc for interlaced scanning.

4. In a television transmitting device comprising a two-dimensional photo-electric mosaic screen in combination means for performing a lighting operation to illuminate said mosaic screen from the object to be transmitted, interrupting means for periodically interrupting the lighting of each element of said mosaic screen, the lighting period allowed by said interrupting means being smaller than an image change period, scanning means for performing a scanning operation to scan said mosaic screen by a ray, and means for establishing a co-operative relation between said interrupting and said scanning means such as to cause said lighting and said scanning operation to act substantially simultaneously on, though at each moment on different parts of, said mosaic screen and such as to admit the passing of at most a small time interval between the end of said lighting operation and the beginning of said scanning operation for each element of said mosaic screen.

5. In a television transmitting device comprising a two-dimensional photo-electric mosaic screen in combination means for performing a lighting operation to illuminate said mosaic screen from the object to be transmitted, interrupting means comprising a rotating apertured diaphragm disposed between the place of said object to be transmitted and said mosaic screen for periodically interrupting the lighting of each element of said mosaic screen, the lighting period allowed by said interrupting means being smaller than an image change period, scanning means for performing a scanning operation to scan said mosaic screen by a ray, and means for establishing a co-operative relation between said interrupting and said scanning means such as to cause said lighting and said scanning operation to act substantially simultaneously on, though at each moment on different parts of, said mosaic screen and such as to admit the passing of at most a small time interval between the end of said lighting operation and the beginning of said scanning operation for each element of said mosaic screen.

6. In a television transmitting device comprising a two-dimensional photo-electric mosaic screen in combination means for performing a lighting operation to illuminate said mosaic screen from the object to be transmitted, interrupting mean comprising a rotated apertured diaphragm' disposed between the place of said object to be transmitted and said mosaic screen ior periodically interrupting the lighting of each element of said mosaic screen, the lighting period allowed by said interrupting means being smaller than, an image change period, scanning means for performing a scanning operation to scan said mosaic screen by a. ray, optical means for producing in the plane oi said diaphragm a real image of said object to be transmitted,

' further optical means for producing on said screen an image of any aperture of said diaphragm which, in the movement of said diaphragm, happens to be situated in the path of the light projected from the mentioned object onto said screen, and means for establishing a co-operative relation between said interrupting and said scanning .means such as to cause said lighting and said scanning operation to act substantially sin1ultaneously on, though at each moment on different. parts of, said mosaic screen and such as to admitthe passing of at most a small time interval along which said screen is scanned, and wherein means are provided for rotating said diaphragm in that sense, with which the lighting ray passing through said apertures moves across said screen in the same direction in which the lines of the ray scanning follow one on the other.

8. The invention set forth in claim 5, wherein said rotating, diaphragm is provided with a plurality of apertures which have, similarly to the holes of a Nipkow disc, a spiral distribution over said diaphragm.

9. In a television transmitting device comprising apparatus for intermittently advancing a cinematographic film and a two-dimensional photo-electric mosaic screen in combination means for performing a lighting operation to illuminate said mosaic screen from said film, interrupting means comprising a rotating apertured diaphragm disposed between the place of the film-picture to be transmitted and said mosaic screen for periodically interrupting the lighting ofeach element of said mosaic screen, the lighting period allowed by said interrupting means being smaller than an image change period, scanning means for performing a scanning operation to scan said mosaic screen by a ray, means for blocking said ray during a time interval which is the sum of the time required for one advancing movement of the film and of said lighting period, and means for establishing a cooperative relation between said interrupting and said scanning means such as to cause said lighting and said scanning operation to act substantially simultaneously on, though at each moment on different parts of, said mosaic screen and such as to admit the passing of at most a small time interval between the end of said lighting operation and the beginning of said scanning operation for each element of said mosaic screen.

I-LANNS-HEINZ WOLFF. 

